Photorespiration and Internal CO2 Accumulation in Chara corallina as Inferred from the Influence of DIC and O2 on Photosynthesis

An O(2) electrode system with a specially designed chamber for ;whorl' cell complexes of Chara corallina was used to study the combined effects of inorganic carbon and O(2) concentrations on photosynthetic O(2) evolution. At pH = 5.5 and 20% O(2), cells grown in HCO(3) (-) medium (low CO(2), pH >/= 9.0) exhibited a higher affinity for external CO(2) (K((1/2))(CO(2)) = 40 +/- 6 micromolar) than the cells grown for at least 24 hours in high-CO(2) medium (pH = 6.5), (K((1/2))(CO(2)) = 94 +/- 16 micromolar). With O(2) </= 2% in contrast, both types of cells showed a high apparent affinity (K((1/2))(CO(2)) = 50 - 52 micromolar). A Warburg effect was detectable only in the low affinity cells previously cultivated in high-CO(2) medium (pH = 6.5). The high-pH, HCO(3) (-)-grown cells, when exposed to low pH (5.5) conditions, exhibited a response indicating an ability to fix CO(2) which exceeded the CO(2) externally supplied, and the reverse situation has been observed in high-CO(2)-grown cells. At pH 8.2, the apparent photosynthetic affinity for external HCO(3) (-) (K((1/2))[HCO(3) (-)]) was 0.6 +/- 0.2 millimolar, at 20% O(2). But under low O(2) concentrations (</=2%), surprisingly, an inhibition of net O(2) evolution was elicited, which was maximal at low HCO(3) (-) concentrations. These results indicate that: (a) photorespiration occurs in this alga and can be revealed by cultivation in high-CO(2) medium, (b) Chara cells are able to accumulate CO(2) internally by means of a process apparently independent of the plasmalemma HCO(3) (-) transport system, (c) molecular oxygen appears to be required for photosynthetic utilization of exogenous HCO(3) (-): pseudocyclic electron flow, sustained by O(2) photoreduction, may produce the additional ATP needed for the HCO(3) (-) transport.